A hole saw is generally composed of a tubular body having a cutting knife provided at its brim, and a guide drill extending from inside the tubular body to the end part, and by rotating the both together, a hole nearly equal to the outside diameter of the tubular body is drilled in the workpiece while being guided by the guide drill. At this time, as a matter of course, the hole drilled in the workpiece is circular. Therefore, when drilling a circular hole, it was widely believed among the manufacturers in the field of this art that the shape of the tubular body of the hole saw should be preferably circular.
However, when drilling a circular hole by a tubular body of cylindrical shape, the gap between the drilled hole and the tubular body is extremely slight. Therefore, cutting chips pose a serious problem. For example, if the workpiece is a soft iron and when its thickness is over 10 mm, or in the case of ALC plate which generates many cutting chips, the problem becomes manifest.
To solve this chip problem, generally, it is known to form a spiral groove for removing cutting chips on the outer circumference of the tubular body. This groove is, however, generally formed by cutting, which complicates the manufacturing process of hole saw. To discharge chips, the required depth of groove is about 1.0 to 1.5 mm or more, and to form such groove, it is necessary to form the tubular body in a thickness of about 3.0 mm. Therefore, complication of manufacturing process and increase of wall thickness cause to increase the product cost.